Normally electrical resistors can be used in most applications without further protection. Usually, resistors are attached to terminal strips by soldering their connecting leads or, alternatively, high power resistors are mounted at each end by metallic retaining clips. When used in a high voltage environment, further improvement of the electrical insulation in the vicinity of the resistor is often added by oil immersion, or, more recently, by the use of insulating gases, such as sulphur hexafluoride, at higher pressure. This simple approach is inadequate for long chains of resistors which are used as voltage dividers in electrostatic particle accelerators.
Typically, an electrostatic particle accelerator with a 10 megavolt terminal potential will have two series strings, each of 300 resistors between the high voltage terminal and ground potential. At the junction of each resistor with its neighbor, connection is made to one of the accelerating elements of the accelerator. Under steady state conditions, the potential difference across the ends of each resistor will be equal to the terminal potential, divided by the number of resistors in the dividing chain. It, therefore, seems trivial to select from resistor manufacturer's charts resistors with working potential differences equal to or greater than the potential difference per resistor, as calculated above.
However, electrostatic accelerators develop spark discharges, both local to a few elements of the accelerating structure (i.e., between one of two accelerating electrodes), or in general to the whole accelerator by means of a spark from the high voltage terminal to the ground potential wall of the surrounding pressure vessel. During spark discharges, the uniform division of potential from terminal to ground is interrupted, and the transient potential division is determined by the capacitive and inductive structure of the accelerator column. Two main processes occur during spark discharges. One is that, since most accelerators are made from cylindrical insulating columns surrounded by a cylindrical metallic pressure vessel, their behavior is that of a coaxial cable shorted at the ground potential end. When a terminal to vessel discharge occurs, a wave then propagates down the column to the ground end where, due to the column-pressure vessel short circuit, it is reflected with opposite polarity, and returns once more to the terminal. As the terminal is, from the radio frequency viewpoint, an open circuit, the wave then reflects at the same polarity towards the tank end. This process continues until the energy stored in the electric field of the accelerator is dissipated as heat in the various protective spark gaps.
The second process, not appreciated in the early days of accelerator design, is that the spark represents a very rapidly changing current flow, typically 50,000 amps in one microsecond. This current flow generates an intense field of radio frequency electromagnetic energy, which produces large transient potentials across the conducting elements of the mechanical structure, which normally would be regarded as equipotentials. In particular, the electromagnetic energy pulse can cause large transient potentials to appear between the turns of the helix of the resistive track of the resistor, which results in turn-to-turn sparking within the resistor. This produces damage to the resistance track.
Protection from these problems was offered in electrostatic particle accelerator draft designs by the use of button spark gaps, which were inherited from the electric power industry. These button spark gap mechanisms were often mounted remote from the resistor it was to protect because it was thought that a metallic connection represented an equipotential and, thus, would protect the resistor adequately. However, this was shown not to be the case, and most electrostatic particle accelerators to this day need replacement of their resistive elements due to overvoltage damage during spark transients.
The first improvement was realized by Daresbury Laboratories in the United Kingdom, which was charged with the development of a superscale accelerator (intended to work at 35 MV terminal potential). The personnel at Daresbury Laboratories found that their grading resistors, mounted in the open and soldered onto lugs mounted at the edges of insulating boards in the power engineering tradition, were destroyed to a large part by a single spark discharge. However, by surrounding the resistors with a conducting screen, this nearly eliminated their problem, even though the screens were not directly associated with the electrical circuits of the resistor. These screens attenuated the radio frequency electromagnetic pulse. Since that time, many different resistor enclosure designs have been produced, with varying degrees of success and most of the new accelerator designs incorporate at least some enclosure for their resistors.
As of the present, the main improvement in resistor design has been to provide a conducting shield to prevent the radio frequency energy from producing large transient potential differences locally along the length of the resistor. These measures have improved the resistor durability many-fold, but have not achieved the lifetimes approaching the steady state lifetimes guaranteed by the manufacturers. For example, upwards of 2,000 resistors are used in large electrostatic particle accelerators. When all of these resistors have aged (i.e., from new), then statistical failure will occur in the example given at 2,000 times the failure rate of a single resistor.
Failure can occur in two ways; namely, the sudden change of the resistor to an open circuit, often accompanied by mechanical disintegration, and the gradual change in resistance value, which distorts the potential distribution along the length of the electrostatic particle accelerator. From keeping records of sparkover, it is obvious that damage is still being produced by spark discharge of the accelerator, as the accelerators operated conservatively suffer little resistor damage (would produce less useful operation time), while accelerators operated aggressively suffer frequent resistor replacement problems. As other developments in the accelerator technology increase the maximum terminal potential available on an accelerator of a given size (enhancing the maximum stored energy), the problem of resistor damage has again become more acute. Derating (that is employing resistors of larger physical size, and, therefore, with higher manufacturer's rating for the maximum applied potential difference), although helpful, does not completely alleviate the problem of early failure. This palliative technique also increases the space that must be devoted for grading resistors, thus restricting the design of more compact accelerators.